Garry wrote:
With the 723, you can make the reference noise as low as you want, by heavy RC filtering. This applies whether you use its own reference or a better external reference. The 723 also seems to work quite happily with a feedback capacitor from the output to the inverting input, reducing the AC gain to unity. The output noise will then just be the buffer amplifier's input noise voltage, ~5-6nV/rtHz.
If the 723 is "great" it is because its functional blocks are available externally -- it allows you to use the full range of design tricks to bring the noise down pretty close to the internal amplifier's input noise. If one needs even lower noise, there is an embarrassingly rich assortment of op-amps available today with noise figures in the 1 nV/rtHz neighborhood. But if you go this route, make sure to keep circuit impedances low so you get the benefit!
One other thing that I don't think has been mentioned -- when using RC or LC networks to filter the reference noise, make sure your C doesn't have a lot of leakage noise, as many electrolytic capacitors do. I have seen many "noise filter" capacitors in commercial products that actually made the overall circuit noise worse by 3 dB or more, with all components operating nominally.
Best regards, Charles _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
